1. Field of the Invention
The present invention relates to a diversity receiver used for radio communication accompanied by fading.
2. Description of the Related Art
The diversity receiver is well known as a means for alleviating the effect of fading (attenuation of the receiving electric field intensity due to the interference of two or more radio waves arriving through different routes). In recent years, the diversity receiver has been widely used for the radio equipment for the base station of mobile unit communications including especially the automobile telephone and portable telephone which cause the fading due to the movement of a local office or a remote office. The most recent application of the diversity receiver is a mobile station requiring miniaturization of the apparatus.
A most common diversity receiver comprises a plurality of antennas each having a receiving section. Such a diversity receiver is of post-detection synthesis type in which the output of each receiving section is selected or synthesized to produce a receiving output under appropriate conditions (such as a high receiving electric field intensity). The post-detection synthesis-type diversity receiver, however, requires a plurality of receiving sections and therefore is complicated in configuration. This inevitably increases the receiver size. In order to simplify the apparatus, a diversity receiver of antenna-switching type with a single receiving section and a plurality of antennas has been suggested which receives radio waves by appropriately switching the antennas under proper conditions.
The configuration of a first example of the conventional diversity receiver of post-detection synthesis type is shown in the block diagram of FIG. 1. In FIG. 1, reference numerals 10, 20 designate antennas, and numerals 30, 40 denote receiving sections for receiving the desired radio wave through the antennas 10, 20 respectively. These receiving sections 30, 40 are capable of detecting the instantaneous value of the receiving electric field intensity. Numeral 50 designates means for comparing the instantaneous values of the receiving electric field intensity obtained from the receiving sections 30, 40, and numeral 60 denotes means for selecting the output of the receiving sections 30 or 40 whichever is larger in the instantaneous value of the receiving electric field intensity as a demodulation output in accordance with the output of the receiving electric field intensity comparator 50.
Explanation will be made below about the operation of the first example of the conventional diversity receiver of post-detection synthesis type configured as described above.
Fading occurs with the movement of a transmitting station or a vehicle equipped with the receiver. As far as a sufficient distance is secured between the antenna 10 and the antenna 20, the correlation between the antennas 10 and 20 is smaller, and therefore the probability is high that the instantaneous values of the receiving electric field intensity at the receiving section 30 and the receiving section 40 are different.
Generally, the output of the receiving section with a higher receiving electric field intensity has lower noise and a better receiving signal. The selector 60 therefore selects the output of the larger instantaneous value of the receiving electric field intensity between the receiving sections 30, 40 based on the output of the receiving electric field intensity comparator 50.
In the first example of the conventional diversity receiver of post-detection synthesis type described above, however, a plurality of receiving sections 30, 40 are required as shown in FIG. 1, thereby leading to the problems of bulkiness and high production cost.